A riverbank has to be a closed area. Your riverbank is not closed but consists of multiple ways, some are even overlapping each other (e.g. way 232583521 and way 232584270). Try to re-edit the area and combine those ways to a single closed loop.

The way was again not correctly closed (between node 2465534058 and node 2480945078), I fixed it for you. JOSM's validator is able to identify such issues quickly. If you are not familiar with JOSM you can use keepright instead but it needs some time to update. By the way keepright also shows several other issues in your area, maybe you want to fix them.